1. Field of the Invention
The present invention relates to tracking induction system and method for optical disc drivers (ODDs), which are capable of seeking a tracking enable position after completing a seek operation for a target track, to perform a tracking operation at the target track,
2. Description of the Prior Art
Generally, detection of data in ODDs is achieved by performing a track seek operation for seeking a desired track and then a tracking operation for making laser beam follow a target track, as disclosed in U.S. Pat. No. 5,090,002. For carrying out the tracking operation after completing the seeking operation, it is required to seek a position enabling the tracking operation. To this end, a tracking induction is carried out. The tracking induction is initiated by deriving a point of time when the tracking induction begins at 1/8 track pitch position before a target track. When a position of laser beam is shifted from a desired position, namely, 1/8 track pitch position before the target track, at the point of time when the tracking induction is carried out, due to a radial run-out of disc and a deflection of a spindle motor, a failure of the tracking induction may be caused. In particular, the shift by over 1/4 track pitch makes it impossible to achieve the tracking induction.
General techniques for such a tracking induction are based on a principle illustrated in FIGS. 1a and 1b. The surface profile of an optical disc is of a structure having lands and grooves alternatingly arranged, as shown in FIG. 1a. One of the lands is predetermined as a target track TT, to carry out a track seek operation. When laser beam is approaching the target track TT, it is required to perform a tracking induction operation wherein the laser beam is positioned at the track center line of target track TT by accurately inducting the laser beam into the target track at a point of tracking induction time.
Here, the tracking induction operation means that the tracking operation which is shut off at a seek mode is initiated when the laser beam is recognized as reaching the target track TT.
Where the tracking induction point for initiating the tracking induction operation is positioned within 1/4 track pitch from the target track TT, in particulate when it is positioned within 1/4 track pitch before the target track TT, a tracking error signal TES which is generated by a detector for servo control of general ODDs exhibits a polarity of negative (-), as shown in FIG. 1b. As a result, the laser beam is shifted rightward so that it is positioned at the track centerline of target track TT. On the other hand, when the tracking induction point is positioned within 1/4 track pitch behind the target track TT, the tracking error signal TES exhibits a polarity of positive (+), so that the laser beam is shifted leftward, so as to be positioned at the track center line of target track TT. Where the tracking induction point is positioned within a range of 1/4 track pitch to 1/2 track pitch before the target track TT, the laser beam is shifted rightward, since the tracking error signal TES has the negative polarity. In this case, however, a positive feedback effect occurs, that the tracking error value increases as the laser beam is shifted. Due to such a positive feedback effect, the laser beam does not reach the target track TT, but jumps. Then, the operation for seeking the target track TT is carried out again. As a result, a long time is required for making the laser beam reach the target track TT, in that there is required the time taken to read information about the target track TT for checking whether the object track corresponds to the desired target track TT and the track jumping time, namely, the time taken to seek the target track TT. Where the tracking induction point TIP is the point A indicated in FIG. 1b, the laser beam is shifted leftward since the polarity of tracking error signal TES is positive, so that it can not reach the target track TT, but moves to the point B. In some cases, the laser beam may be positioned at one track pitch before the target track TT.